1. Field of the Invention
The present invention relates to a method of fabricating a thin film transistor using dual or multiple gates, and more particularly, to a method of fabricating a thin film transistor using dual or multiple gates which is capable of improving uniformity of the thin film transistor using a polycrystalline silicon thin film by using the dual or multiple gates.
2. Description of the Related Art
Bonding defects such as atom dangling bonds existing on the crystal grain boundaries of polycrystalline silicon included in active channel regions are known to act as traps on electric charge carriers when fabricating a thin film transistor (hereinafter referred to as a TFT) using polycrystalline silicon.
Therefore, size, size uniformity, number and position, and direction of crystal grains not only directly or indirectly exert a fatal influence on the TFT characteristics such as threshold voltage (Vth), subthreshold slope, charge carrier mobility, leakage current and device stability, but also exert a fatal influence on uniformity of the TFT depending on the position of the crystal grains when fabricating an active matrix display substrate using the TFT.
The number of fatal crystal grain boundaries (hereinafter referred to as “primary” crystal grain boundaries) included in active channel regions of a TFT on the whole substrate of a display device can be equal to or different from each other depending on the size of crystal grains, the inclination angle Θ, the dimension of active channels (length (L), width (W)) and position of each TFT on the substrate, as illustrated in FIG. 1A and FIG. 1B.
As illustrated in FIG. 1A and FIG. 1B, the number of “primary” crystal grain boundaries that can be contained in an active channel region for the crystal grain size Gs, the active channel dimension L×W and the inclination angle Θ is Nmax (2 in case of FIG. 1A) or (Nmax−1) (3 in case of FIG. 1B) when the number of the maximum crystal grain boundaries is Nmax, and the most superior uniformity of the TFT characteristics can be obtained when Nmax “primary” crystal grain boundaries are contained in the active channel region for all TFTs. Accordingly, the more each of the TFTs have an equal number of crystal grain boundaries, the more superior uniformity a device has.
On the other hand, it can be easily expected that uniformity is the worst in characteristics of TFTs on a TFT substrate or a display device if the number of TFTs including Nmax “primary” crystal grain boundaries is equal to the number of TFTs including (Nmax−1) “primary” crystal grain boundaries.
Polycrystalline or single crystalline particles can form large silicon grains on a substrate using sequential lateral solidification (SLS) crystallization technology, as illustrated in FIG. 2A and FIG. 2B. It has been reported that a TFT fabricated using the large silicon grains can obtain the similar characteristics to that of a TFT fabricated using single crystalline silicon.
However, numerous TFTs used in a driver and a pixel array should be fabricated in order to fabricate an active matrix display.
For example, approximately a million pixels are required in fabricating an active matrix display having super video graphics array (SVGA) resolution, one TFT is required in each pixel in the case of a liquid crystal display (LCD), and two or more TFTs are required in each pixel in a display device using an organic light emitting substance, e.g. an organic electroluminescent device.
Therefore, it is impossible to fabricate the TFTs by growing a certain number of crystal grains only in one to two million or more active channel regions of each TFT in a certain direction.
In order to supplement the problems, it is disclosed in PCT International Patent NO. WO 97/45827 that the amorphous silicon on the whole substrate is converted into polycrystalline silicon, or only selected regions on the substrate are crystallized using SLS technology after depositing amorphous silicon by plasma enhanced chemical vapor deposition (PECVD), low pressure chemical vapor deposition (LPCVD) or sputtering (FIG. 2A and FIG. 2B).
The selected regions are also considerably wide regions compared with active channel regions having a dimension of several μm×several μm. Furthermore, the size of a laser beam used in SLS technology is approximately several μm×several μm so that stepping and shifting of the laser beam or the stage are inevitably required to crystallize amorphous silicon of the whole regions or selected regions on a substrate, wherein misalignment exists between regions on which the laser beam is irradiated. Therefore, the number of “primary” crystal grain boundaries included in numerous active channel regions of a TFT is varied, and the TFT on the whole substrate, in driver regions or in pixel cell regions has unpredictable non-uniformity. The non-uniformity can exert a fatal influence on the realization of an active matrix display device.
Furthermore, it is disclosed in U.S. Pat. No. 6,177,301 that the barrier effect of crystal grain boundaries for the direction of an electric charge carrier is minimized (FIG. 3A), and TFT characteristics being second to single crystalline silicon is obtained accordingly in the case where the direction of active channels is parallel to the direction of crystal grains grown by the SLS crystallization method when fabricating a TFT for an LCD comprising driver and pixel arrays by forming large silicon grains using SLS crystallization technology while a lot of crystal grain boundaries in which the TFT characteristics acts as a trap for the electric charge carriers exist, and the TFT characteristics is greatly deteriorated in the case where the active channel direction is perpendicular to the crystal grain growing direction (FIG. 3B).
There are cases where TFTs inside the driver circuit and TFTs inside pixel cell regions usually have an angle of 90° when actually fabricating an active matrix display, wherein uniformity of the device can be improved by fabricating the active matrix display in such a way that a direction of the active channel region is inclined at a growing angle of the crystal grains at an angle of 30 to 60° to improve uniformity of characteristics between TFTs, while not greatly deteriorating characteristics of each TFT as illustrated in FIG. 3C.
However, there is a probability that fatal crystal grain boundaries are included in the active channel regions as the method also uses crystal grains of a limited size formed by the SLS crystallization technology. Accordingly, the method has problems in that unpredictable non-uniformity causing a difference of characteristics between TFTs exists.